Researchers have succeeded in making a model of breathing movement that allows for the precise measurement of narrow beams to a dummy tumor by simulating the motion and physical properties of the chest anatomy in a model. This research was presented at the 3rd ESTRO (European Society for Therapeutic Radiology and Oncology) Forum in Barcelona, Spain.

Radiotherapy using protons can deliver more accurate treatment to a tumor while reducing the dose to surrounding tissue. However, in mobile organs such as the lung, precise targeting of the dose is difficult.

Rosalind Perrin, PhD, from the Centre for Proton Therapy at the Paul Scherrer Institute, Villigen, Switzerland, described the method she and colleagues have developed to test the application of proton therapy to lung cancer. The method uses a delivery technique called rescanning, which helps to mitigate the effect of motion. The researchers are also working to develop practical ways to implement it in the clinic for patient treatments.

"This involved experiments using an advanced breathing model of the patient, a so-called anthropomorphic phantom, with integrated measurement devices to accurately measure the dose distribution. We found that our rescanning technique worked well to overcome the effect of motion on the dose delivered to the tumor, and for tumor motions of up to 1 cm," she said.

The model developed by the researchers was made up of a sphere representing a tumor moving within an inflating lung, enclosed in a rib cage complete with surrounding muscle and skin layers. The model can be programmed to move with breathing patterns specific to each patient.

Radiation dosage was measured during movement, and the researchers found that the rescanning technique allowed the application of clinically acceptable dose distribution to the tumor, and only a minimal dose to surrounding tissues.

Scanning proton therapy is an emerging technology in cancer therapy, in which a narrow particle beam, consisting of accelerated hydrogen nuclei, is scanned through the tumor and administers highly targeted radiation to the cancer cells.

Because protons have a relatively large mass, the beam delivers most of its radiation dose towards the end of its path in tissue, and thus proton therapy can be designed to limit dose to surrounding tissues. Furthermore, a proton beam only penetrates the tissue up to a given depth, determined by its energy. So, compared with conventional radiotherapy techniques, the therapy allows a maximal dose to the tumor, while reducing the dose elsewhere.

However, for mobile tumors in the liver or lung, organ and tumor motion deteriorates the dose distribution because there may be a rift between the radiation delivery time-line and the time-line of the tumor motion: the interplay effect. The researchers have worked to overcome this problem by developing a new, state-of-the art delivery system, and the technology required by these advanced motion mitigation methods is now operational. The rescanning technique involves scanning the tumor several times by the proton beam.

"This makes it possible to average out the dose to the moving tumor, and also reduce the effect of motion on the dose delivered to it. Because of the sensitivity of the lung to radiation, as well as the proximity of the heart, esophagus, and spinal cord, it is particularly important to keep the radiation dose to surrounding tissues as low as possible in lung cancer," said Perrin.